When you attempt to access another users mailbox, or schedule, even if you have been granted access, the following event is logged:
Event ID: 1016 Source: MSExchangeIS Private Type: Success Audit Category: Logons NT User DomainName\UserName logged on to John/Jane Doe mailbox, and is not the primary Windows NT account on this mailbox.An Event ID 1016 is logged, even if you were not successful.
Some antivirus software requires that you use an Exchange Service Admin account, so it can open and scan all mailboxes. This causes Event ID 1016 to be logged for each mailbox, with a text similar to:
NT User ExchService logged on to ExUser<n> mailbox, and is not the Primary Windows NT Account on this mailbox